1. Field of Invention:
The present invention relates to apparatus for controlling the depth of marine cables.
2. Description of Prior Art:
Remote controlled seismic cable depth controllers have in the past been of two general types or categories.
A first type, such as in U.S. Pat. Nos. 3,412,704; 3,541,989; and 3,605,674 have been based on controlling the depth of depth controllers or paravanes disposed at spaced positions on the cables by means of signals sent from transmitters mounted in the seismic cable near each controller to receivers in the controllers. Components of the controller adjusted the depth of the cable in response to the signals received at the receiver. These types of apparatus thus required a special cable for remote control seismic surveying, containing transmitters at spaced locations therein. Since typical seismic cables may reach one or two miles in length for survey operations, it was undesirable for economic reasons to require a special cable, different from that normally used, for remote control seismic surveying.
A second type of cable depth controller has used frequency responsive sensors in the depth controller to receive signals sent through a conductor in the cable (as in U.S. Pat. Nos. 3,648,642 and 3,673,556) or through the water (as in U.S. Pat. No. 3,412,704). However, the number of depths to which the controllers could be raised or lowered depended on the number of frequencies used. As the number of frequencies increased, the number of frequency-sensitive filters or frequency-sensitive relays in the controllers increased. The sensitivity of the frequency-sensitive members has to increase, so far as is known, if several frequencies were attempted for several different depths. Also, since analog signals were used with cycles of frequency specifying the desired depth, and these frequencies could drift, depth controllers could deviate from the desired depth settings and affect cable depth.